Dither mask selection for printing using aligned printheads

ABSTRACT

A method of printing an image using a printing system having first and second printheads supplied with a same ink. The method includes the steps of: allocating first lines of the image to the first printhead; allocating second lines of the image to the second printhead; dithering the first lines of the image using a selected first dither mask; dithering the second lines of the image using a selected second dither mask; printing the dithered first lines of the image using the first printhead; and printing the dithered second lines of the image using the second printhead. The first and second dither masks are selected based on a relative alignment of the first and second printheads.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation application of U.S.application Ser. No. 17/206,720 filed Mar. 19, 2021, which claims thebenefit of priority under 35 U.S.C. § 119(e) of U.S. ProvisionalApplication No. 62/992,672 filed Mar. 20, 2020, the contents of whichare hereby incorporated by reference in their entirety for all purposes.

FIELD OF THE INVENTION

The present invention relates to a method of dithering for high-speedsingle-pass printing. It has been developed primarily for optimizingprint quality when printing using multiple monochrome printheads alignedalong a media feed direction.

BACKGROUND

Single-pass (“pagewide”) printing dramatically increases print speedscompared to traditional scanning printheads. One application ofsingle-pass printing is in digital inkjet presses, as described in, forexample, U.S. Pat. Nos. 10,457,075; 10,081,204 and 9,421,790, thecontents of which are incorporated herein by reference.

In a typical color inkjet press, multiple monochrome printheads arealigned with each other along a media feed direction with each printheadprinting a different color (e.g. CMYK). In principle, print speeds maybe increased by increasing the number of printheads used to print eachcolor. For example, a monochrome black printing system may double itsprinting speed by doubling the number of printheads printing black ink—afirst printhead may print half of the dots required by a halftone imageand a second downstream printhead may print the other half of the dots.It will be appreciated that print speeds may be tripled, quadrupled etc.by further increasing the number of aligned printheads.

As is well known in the art, halftone images (or bitmaps) containingdiscrete dots at a specified printing resolution are generated fromcontone images using a dithering process. For printing at double speedusing two printheads, simplistically, the halftone image needs to bedivided (“deinterleaved”) into two portions, one for each printhead. Forexample, alternate lines of the halftone image may be printed by each ofthe two printheads—the first printhead printing odd lines of the imageand the downstream second printhead printing even lines of the image orvice versa. Thus, each printhead receives a halftone image at half theresolution (in the printing direction) of the full halftone image. If,for example, the full halftone image is generated at a resolution of1600×790 dpi, then each printhead would receive a respective halftoneimage at a resolution of 1600×395 dpi. The two halftone images areinterleaved by the two printheads during printing to recreate the fullhalftone image at the target resolution of 1600×790 dpi.

Printing at increased speeds using multiple high-resolution printheadsin the manner described above requires excellent alignment of theprintheads. Without excellent alignment of the printheads, print qualitygenerally declines to an extent that is unacceptable to users.

Misalignment of printheads may arise from a number of sources. Firstly,mechanical placement of the printheads must be accurately controlled towithin a few microns in order to avoid misalignment between a pair ofprintheads. The Applicant has previously described sophisticatedalignment patterns (see U.S. application Ser. No. 17/736,348 filed 7Jan. 2020, the contents of which are incorporated herein by reference),which may be used to determine misalignments and allow fine correctionthereof using suitable mechanical means. However, even with perfectlymechanically positioned printheads, relative misalignments are stillpossible due to the small differences in the conformation of individualprintheads, arising from their manufacturing process. For example, inrelatively long pagewide printheads, the printheads have a tendency tobow along their long axis, some more than others depending on theprecise manufacturing conditions. If a relatively ‘bowed’ printhead ispaired with a relatively ‘non-bowed’ printhead, then those twoprintheads will typically have poor alignment even with perfectmechanical positioning of the two printheads. Whilst studies by theApplicant show that only a relatively small percentage (about 10%) ofrandomized printhead pairings are unmatched, ideally, it should beensured that ‘non-bowed’ printheads are matched with similarly‘non-bowed’ printheads and ‘bowed’ printheads are matched with similarly‘bowed’ printheads in order to maximize alignment. However, printheadmatching in this way is problematic in a supply chain. Users expectprintheads to be replaceable with any printhead of the same type;moreover, measuring an extent of bowing and then categorizing printheadsbased on such measurements adds complexity and expense to the supplychain. Even with such a categorization of printheads at the factory, theextent of bowing may change due to other factors in the field. Forexample, a printhead nest which mechanically holds the printhead wheninstalled in a printing system may affect the extent of bowing. Hence,characterization of printhead bowing at the factory may not be a goodindicator of printhead bowing in the field.

It would therefore be desirable to provide a method of high-speedprinting using upstream and downstream printheads, which optimizes printquality irrespective of the relative alignment of those printheads. Inparticular, it would be desirable to optimize print quality forprintheads that are not perfectly aligned, either due to imperfectmechanical placement of the printheads or mismatching of printheadpairings.

SUMMARY OF INVENTION

In a first aspect, there is provided a method of single-pass printingusing a printing system comprising at least first and second alignedprintheads supplied with a same ink, the second printhead beingdownstream of the first printhead, the method comprising the steps of:receiving first and second halftone images at the first and secondprintheads, respectively; printing the first halftone image from thefirst printhead; and

printing the second halftone image from the second printhead such that aresulting printed image contains the second halftone image interleavedwith the first halftone image, wherein:

the printed image is printed at a first resolution in a printingdirection;

the first and second halftone images have a second resolution in theprinting direction, the second resolution being less than the firstresolution;

the first halftone image is based on a first dither pattern and thesecond halftone image is based on a second dither pattern, the firstdither pattern being different than the second dither pattern.

Preferably, the method comprises the steps of providing a full contoneimage for one ink channel of the printing system at the first resolutionin a printing direction.

Preferably, the method comprises the steps of:

dividing the full contone image at the first resolution into at leastfirst and second contone images at the second resolution in the printingdirection, the second resolution being less than the first resolution;

dithering the first contone image using the first dither pattern toprovide the first halftone image at the second resolution;

dithering the second contone image using the second dither pattern toprovide the second halftone image at the second resolution.

Preferably, the first and second contone images contain respective firstand second sets of lines of the full contone image.

Preferably, the first and second contone images contain respectivealternate lines of the full contone image.

Preferably, the first and second contone images have a same resolutionas the full contone image in a direction perpendicular to the printingdirection.

Preferably, the method comprises the steps of:

dithering the full contone image using a combined dither pattern toprovide a full halftone image at the first resolution;

dividing the full halftone image into at least first and second halftoneimages at the second resolution in the printing direction, the secondresolution being less than the first resolution;

wherein the combined dither pattern is a combination of a first ditherpattern for the first printhead and a second dither pattern for thesecond printhead, the first dither pattern being different than thesecond dither pattern.

Preferably, the first and second halftone images contain respectivefirst and second sets of lines of the full halftone image.

Preferably, the first and second halftone images contain respectivealternate lines of the full halftone image.

Preferably, the step of providing the contone image for one ink channelcomprises one or more of:

rasterizing;

calibrating to a target printing resolution; and

generating, from a color contone image, a plurality of contone imagescorresponding to respective ink channels of the printing system.

Preferably, the printing system comprises n aligned printheads suppliedwith the same ink and the second resolution is 1/n of the firstresolution in the printing direction.

Preferably, the printing system comprises two aligned printheadssupplied with the same ink and the second resolution is half of thefirst resolution.

Preferably, the first and second dither patterns are independentlyselected from the group consisting of: blue noise dither patterns andgreen noise dither patterns.

In a related aspect, there is provided a print medium having an imageprinted thereon, the image comprising a first halftone image interleavedwith a second halftone image, the first and second halftone images beingprinted with a same ink, wherein the first halftone image is based on afirst dither pattern and the second halftone image is based on a seconddither pattern, the first dither pattern being different than the seconddither pattern.

In a second aspect, there is provided a method of processing a contoneimage for single-pass printing using a printing system comprising atleast first and second aligned printheads supplied with a same ink, thesecond printhead being downstream of the first printhead, the methodcomprising the steps of:

providing the contone image for one ink channel at a first resolution ina printing direction;

dithering the contone image using a combined dither pattern to provide afull halftone image at the first resolution;

dividing the full halftone image into at least first and second halftoneimages at a second resolution in the printing direction, the secondresolution being less than the first resolution; and

sending the first and second halftone images to respective first andsecond printheads for printing;

wherein the combined dither pattern is a combination of a first ditherpattern for the first printhead and a second dither pattern for thesecond printhead, the first dither pattern being different than thesecond dither pattern.

Preferably, the first and second halftone images contain respectivefirst and second sets of lines of the full halftone image.

Preferably, the first and second halftone images contain respectivealternate lines of the full halftone image.

Preferably, the first and second halftone images have a same resolutionas the full halftone image in a direction perpendicular to the printingdirection.

Preferably, the printing system comprises n aligned printheads suppliedwith the same ink and the second resolution is 1/n of the firstresolution in the printing direction.

Preferably, the printing system comprises two aligned printheadssupplied with the same ink and the second resolution is half of thefirst resolution.

Preferably, the step of providing the contone image for one ink channelcomprises one or more of:

rasterizing;

calibrating to a target printing resolution; and

generating, from a color contone image, a plurality of contone imagescorresponding to respective ink channels of the printing system.

Preferably, the method further comprises the step of:

printing the first halftone image from the first printhead; and

printing the second halftone image from the second printhead such that aresulting printed image contains the second halftone image interleavedwith the first halftone image, wherein the first and second halftoneimages are printed at the second resolution in the printing direction.

Preferably, the first and second dither patterns are independentlyselected from the group consisting of: blue noise dither patterns andgreen noise dither patterns.

In a related aspect, there is provided a processor configured to performthe steps of:

receiving or generating a contone image for one ink channel of aprinting system at a first resolution in a printing direction;

dithering the contone image using a combined dither pattern to provide afull halftone image at the first resolution;

dividing the full halftone image into at least first and second halftoneimages at a second resolution, the second resolution being less than thefirst resolution;

sending the first and second halftone images to respective first andsecond printheads for printing;

wherein the combined dither pattern is a combination of a first ditherpattern for the first printhead and a second dither pattern for thesecond printhead, the first dither pattern being different than thesecond dither pattern.

In another related aspect, there is provided a printing systemcomprising:

the processor as described above; and

at least first and second aligned printheads,

wherein the second printhead is downstream of the first printhead.

In another related aspect, there is provided a method of processing acontone image for single-pass printing using a printing systemcomprising at least first and second aligned printheads supplied with asame ink, the second printhead being downstream of the first printhead,the method comprising the steps of:

providing the contone image for one ink channel at a first resolution ina printing direction;

selecting a dither pattern from the group consisting of: (a) a combineddither pattern comprising a combination of a first dither pattern forthe first printhead and a second dither pattern for the secondprinthead, the first dither pattern being different than the seconddither pattern; and (b) a third dither pattern for the first and secondprintheads;

dithering the contone image using the selected dither pattern to providea full halftone image at the first resolution;

dividing the full halftone image into at least first and second halftoneimages at a second resolution in the printing direction, the secondresolution being less than the first resolution; and

sending the first and second halftone images to respective first andsecond printheads for printing,

wherein selection of the dither pattern is based on relative printheadalignment between the first and second printheads.

Preferably, the combined dither pattern is selected for relativelypoorly aligned first and second printheads and the third dither patternis selected for relatively well aligned printheads.

Preferably, the relative printhead alignment is based on qualitativeprint quality feedback from a user.

Preferably, the relative printhead alignment is based on a quantitativealignment measurement.

Preferably, the dither pattern is selected automatically, the selectionbeing based on the quantitative alignment measurement relative to apredetermined threshold.

Preferably, the third dither pattern is the same or different than thefirst dither pattern or the second dither pattern.

Preferred embodiments of the invention described above in relation toone aspect are, of course, equally applicable to other aspects of theinvention where relevant.

As used herein, the term “ink” is taken to mean any printing fluid,which may be printed from an inkjet printhead. The ink may or may notcontain a colorant. Accordingly, the term “ink” may include conventionaldye-based or pigment-based inks, infrared inks, fixatives (e.g.pre-coats and finishers), 3D printing fluids and the like. Wherereference is made to fluids or printing fluids, this is not intended tolimit the meaning of “ink” herein.

As used herein, the term “aligned printheads” is taken to mean aplurality of printheads that are generally aligned along a media feeddirection, such that any one of the printheads is capable of printingonto a same part of the media as any other printhead. However, as willbe readily apparent to the person skilled in the art, the term “alignedprintheads” should not necessarily be taken to mean perfect alignment,positioning and/or matching of printheads as foreshadowed above. In oneaspect, the present invention addresses the problem of smallmisalignments (e.g. less than 500 micron, less than 200 micron, lessthan 100 micron or less than 50 micron misalignments) between generallyaligned printheads.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the present invention will now be describedwith reference to the drawings, in which:

FIG. 1 is a schematic plan view of a printing system comprising firstand second aligned printheads;

FIG. 2 is a perspective of a print engine comprising four print modules;

FIG. 3 is a schematic plan view of a printing system comprising an 8×2array of printheads;

FIG. 4 shows conventional processing steps for generating first andsecond halftone images for printing using the printing system shown inFIG. 1 ;

FIG. 5 shows processing steps for generating first and second halftoneimages according to a first embodiment; and

FIG. 6 shows processing steps for generating first and second halftoneimages according to a second embodiment.

DETAILED DESCRIPTION

Referring to FIG. 1 , there is shown schematically a printing system 1comprising a first printhead 2 and a second printhead 4 positioneddownstream of the first printhead relative to a media feed directionindicated by arrow F. The second printhead is aligned with the firstprinthead in the media feed direction insofar as both printheads arecapable of printing onto a same portion of media 6 (e.g. cut-sheet mediaor a roll-to-roll fed media web). An extent of alignment between thefirst and second printheads 2 and 4 may vary according to individualprinthead conformations, printhead placement accuracy etc, as describedabove.

Each of the first and second printheads 2 and 4 is a monochromeprinthead supplied with a same ink so as to enable double-speedprinting. For double-speed printing, each printhead prints half an imageat half the target resolution (in the media direction F). For example, afull halftone image may be generated at a target resolution of 1600×790dpi and each of the first and second printheads 2 and 4 is configured toprint at a resolution of 1600×395 dpi. Typically, each printhead printsrespective alternate lines (row) of the full halftone image. Sinceprintheads have a maximum drop ejection frequency, it will beappreciated that halving the resolution in the media feed direction Fenables printing at twice the speed that would otherwise be obtainable.

Each of the first and second printheads 2 and 4, is typically acomponent of a print module, which may additionally comprise a printheadmounting structure, electronics for supply of data and power to theprinthead, ink couplings, pressure regulator(s) etc. Examples ofsuitable print modules are described in U.S. Pat. Nos. 10,457,075 and10,081,204, the contents of which are incorporated herein by reference.

By way of example, and referring to FIG. 2 , there is shown a printengine 10 having four aligned print modules 12 as described in US2019/0118537, the contents of which are incorporated herein byreference. Each print module 12 comprises a respective printhead (notvisible in FIG. 2 ) as well as ink couplings, PCBs etc. It will beappreciated that the print engine 10 having four aligned printheadssupplied with a same ink potentially enables quadruple-speed printing byallocating a quarter of a full halftone image to each of the four printmodules 12. For example, every 4^(th) line of the full halftone imagemay be allocated to a respective print module 12, such that eachprinthead prints at one ¼ resolution in the media feed direction. A fullhalftone image generated at a resolution of 1600×800 dpi would beprinted at a resolution of 1600×200 dpi by each printhead, therebyenabling the print engine 10 to print at quadruple speed compared to asingle print module 12.

In principle, any number of n printheads may be used to print at n timesspeed by allocated 1/n of a full halftone image (e.g. every nth line ofthe halftone image) to a respective one of the n printheads.

Of course, the aligned printheads may be part of a matrix of printheadsarranged for color and/or wideformat printing. FIG. 3 showsschematically a print engine 20 containing sixteen print modules 12 inan 8×2 array for full color printing. The print modules 12 are arrangedin sets of four for printing each of four colors (KCMY). In each of thecolor channels (KCMY), there are two pairs of aligned print modules 12,each pair of aligned print modules overlapping to print onto a differentportion of the media. For example, as shown in the black channel, thepair of print modules 12A are aligned and the pair of print modules 12Bare aligned along the media feed direction F. Thus, each ink (color)channel is capable of printing at double speed in the manner describedabove in connection with FIG. 1 .

Referring to FIG. 4 , there is shown a simple method of processing acontone image at a first resolution for high-speed printing using thefirst printhead 2 and the second printhead 4 shown in FIG. 1 . Themethod is performed in a raster image processor (RIP) although, for thesake of clarity, not all processing steps performed by the RIP are shownin FIG. 4 . The contone image is a contone (grayscale) bitmap for asingle ink channel at the target printing resolution of 1600×790 dpi.The skilled person will understand that typical upstream processingsteps in the RIP (e.g. rasterizing, color space conversion, ink channelseparation, calibration to the target printing resolution etc) are notshown in FIG. 4 .

Still referring to FIG. 4 , in a first step, the contone image isdithered using a conventional single dither pattern (e.g. a blue noisedither as described in U.S. Pat. No. 5,111,310 or a green noise ditheras described in U.S. Pat. No. 6,493,112 etc.) to generate a fullhalftone image. The full halftone image is then divided into a firsthalftone image and a second halftone image, each at a second resolution,in a process known as “deinterleaving”. Alternate rows of the fullhalftone image are allocated to respective printheads, such that each ofthe first and second halftone images resulting from the deinterleavingprocess has a resolution of 1600×395 dpi. For example, the firsthalftone image may comprise odd lines (1, 3, 5, 7 etc.) of the fullhalftone image and the second halftone image may comprises even lines(0, 2, 4, 8 etc.) of the full halftone image or vice versa. The firstand second halftone images are then sent to respective first and secondprintheads of a print engine and for printing.

In the single-pass printing process using a first printhead 2 and adownstream second printhead 4, as shown in FIG. 1 , the full printedimage 8 contains the first and second halftone images interleaved on themedia 6 to represent the full halftone image generated from thedithering process. The half-density image 9 printed by the upstreamfirst printhead 2 is based on the first halftone image only and istherefore printed at half density (alternate lines of the full halftoneimage).

With perfect alignment of the first printhead 2 and second printhead 4,the process described in connection with FIG. 4 provides excellent printquality and enables double-speed printing compared to a single printheadprinting the full halftone image. However, small misalignments betweenthe first printhead 2 and the second printheads 4 (e.g. resulting fromsmall mechanical misplacements and/or mis-matching of printheadconformations) results in a significant decline in print quality. Inparticular, print quality defects are exacerbated by interferenceeffects between the first and second halftone images, which are nototherwise present when printing one color of ink from one printhead.Increasing misalignments between the first printhead 2 and the secondprinthead 4 result in a rapid decline in print quality, which isgenerally unacceptable to users.

Referring to FIG. 5 , there is shown a method of processing a contoneimage according to a first embodiment of the invention. In the methodshown in FIG. 5 , the contone image is deinterleaved prior to dithering.Deinterleaving of the contone image is performed similarly to thedeinterleaving process described above, whereby alternate lines of thefull contone image are allocated to a first contone image and a secondcontone image. The first contone image is then dithered using a firstdither pattern to generate the first halftone image and the secondcontone image is dithered using a second dither pattern to generate thesecond halftone image. Crucially, the first and second dither patternsare different.

When the first and second halftone images are printed using respectiveprintheads, the printed image 8 generally has acceptable print quality.Advantageously, print quality is relatively tolerant of misalignmentsbetween the first printhead printhead 2 and the second printhead 4compared to the method described above in connection with FIG. 4 . Inparticular, it is understood by the present inventors that usingdifferent dither patterns for the first and second halftone imagesresults in a somewhat less rapid decline in print quality withincreasing misalignments between the printheads when compared to themethod described above using a single dither pattern. Nevertheless, withgood alignment between the first printhead 2 and the second printhead 4,the method according to the first embodiment produces lower printquality than the method described above.

It would be desirable for users to substitute between the two differentprocesses described above in order to optimize print quality fordifferent extents of alignment between printheads. For example, aninitially perfect alignment between the first and second printheads 2and 4 may change over time, or replacement of one or both printheads mayresult in misalignments. In this scenario, it would be desirable tochange from the process shown in FIG. 4 to the process shown in FIG. 5 .However, since the two processes involve different datapaths, it isimpractical to reconfigure the RIP so as to substitute between these twoprocesses.

Referring now to FIG. 6 , there is shown a method of processing acontone image according to a second embodiment. The method shown in FIG.6 has the same datapath as the method shown in FIG. 4 —that is, theordering of processing steps is identical in each case. However, thedithering step in the method shown in FIG. 6 employs a combined ditherpattern comprising the first dither pattern and the second ditherpattern. In other words, alternate lines of the combined dither pattern(or dither mask) are based on different dither patterns. For example,odd lines of the combined dither pattern may be based on the firstdither pattern and even lines of the combined dither pattern may bebased on the second dither pattern, which is different than the firstdither pattern.

Accordingly, dithering using the combined dither pattern results infirst and second halftone images, which are identical to the first andsecond halftone images described above in connection with FIG. 5 . Themethod according to the second embodiment, therefore, enjoys the sameadvantages as the method according to the first embodiment.

Moreover, an additional advantage of the method according to the secondembodiment is that the datapath uses the same sequence of processingsteps as those shown in FIG. 4 . Therefore, by simply substituting aconventional single dither pattern applied to the full contone imagewith the combined dither pattern, the RIP can readily switch betweenthese two methods in order to optimize print quality for a givenscenario.

For example, a user may provide empirical qualitative feedback on printquality and the dither may be switched accordingly. Alternatively, aprinthead alignment test pattern may provide quantitative printheadalignment data, which can be used to select the most appropriate ditherpattern.

The dither pattern may be selected automatically based on a printheadalignment measurement relative to a predetermined threshold. Forexample, if the printheads are determined to be aligned to within onedot pitch or less (in the printing direction) at the resolution of thefirst and second halftone images (i.e. within 64 microns for a 1600×395dpi halftone image), then a single dither pattern may be employed, asshown in FIG. 4 . However, if the printheads are aligned only to greaterthan one dot pitch (in the printing direction) at the resolution of thefirst and second halftone images (i.e. greater than 64 microns for a1600×395 dpi halftone image), then the combined dither pattern may beemployed, as shown in FIG. 6 . The predetermined threshold may bevariable depending on printing parameters (e.g. print speed, print mediatype, ink type etc.). In this way, print quality can be optimized forboth well-aligned and somewhat misaligned printheads.

The foregoing describes only some embodiments of the present invention,and modifications of detail may be made thereto without departing fromthe scope of the invention, the embodiments being illustrative and notrestrictive.

1. A method of printing an image using a printing system having firstand second printheads supplied with a same ink, the second printheadbeing downstream of and aligned with the first printhead, the methodcomprising the steps of: allocating first lines of the image to thefirst printhead; allocating second lines of the image to the secondprinthead; dithering the first lines of the image using a selected firstdither mask; dithering the second lines of the image using a selectedsecond dither mask; printing the dithered first lines of the image usingthe first printhead; and printing the dithered second lines of the imageusing the second printhead, wherein the first and second dither masksare selected based on a relative alignment of the first and secondprintheads.
 2. The method of claim 1, wherein the first and seconddither masks are selected to be different for relatively poorly alignedfirst and second printheads; and the first and second dither masks areselected to be identical for relatively well aligned printheads.
 3. Themethod of claim 1, wherein the relative alignment is based onqualitative print quality feedback from a user.
 4. The method of claim1, wherein the relative alignment is based on a quantitative alignmentmeasurement.
 5. The method of claim 4, wherein the first and seconddither masks are selected automatically based on the quantitativealignment measurement.
 6. The method of claim 1, wherein the first andsecond lines are alternate lines of the image.
 7. The method of claim 1,wherein the first and second dither masks are independently selectedfrom the group consisting of: blue noise dither masks and green noisedither masks.
 8. The method of claim 1, wherein the printing issingle-pass printing, the first and second printheads being stationaryprintheads.
 9. The method of claim 1, wherein the image is divided intofirst and second images prior to dithering, the first image containingonly the first image lines and the second image containing only thesecond image lines.
 10. The method of claim 1, wherein the image isdithered using a combined dither mask, the combined dither maskcomprising the first dither mask for the first image lines and thesecond dither mask for the second image lines.